Clinton highlighted the issue in a Twitter post that linked an article reporting an overnight increase in one drug from $13.50 a tablet to $750 a dose.

"Price gouging like this in the specialty drug market is outrageous," Clinton said. "Tomorrow I'll lay out a plan to take it on."

The biotech index was in the red most of the morning on Wall Street, but losses accelerated sharply after the tweet by Clinton, the leading candidate for the 2016 Democratic Party's presidential nomination.

Clinton attached a link to a New York Times article on the drug Daraprim, which treats a life-threatening parasitic infection. The price skyrocketed after the drug was recently acquired by Turing Pharmaceuticals, a startup founded by former hedge fund investor Martin Shkreli.